Analog Modems or modems used on phone lines used(and still use for that matter) such a technique. I am somewhat rusty on such devices. but try google and the terms "analog modems", or "voiceband modems"
You can also google FSK - Frequency Shift Keying. In this technique a bit cell can be divided into three parts. A zero is a frequency change at the one-third point of the bit cell and a one is a frequency change at the tw-thirds point of the bit cell. The two frequencies are non harmonically related and have a large value for their least common multiple.